A material may be photoemissive if high energy electrons are ejected from the material by photons of electromagnetic radiation by the process known as the photoelectric effect. Vacuum tubes which commonly include a photoemissive cathode or emitter and an anode for collecting electrons emitted from the cathode are generally referred to as phototubes or photodiodes. The number of high energy electrons emitted from the photoemissive material (i.e., the cathode) is generally inversely related to the energy (photons) of the radiation impinging against the material in a predictable relationship. Thus, phototubes may be made and calibrated to measure most forms of electromagnetic radiation.
When a high energy electron (commonly known as a photoelectron or primary electron) is ejected from a photoelectric emissive cathode by photons impinging thereon, a substantial number of low energy secondary electrons may also be ejected from the cathode and collected by the anode. The number of secondary electrons ejected for each photoelectron may not be readily determined by calculation or calibration and, furthermore, may vary during use of the phototube. Thus, the outputs or response of these phototubes may not be predictable in a given radiation field and may be highly nonlinear or energy dependent. At high rates of photoelectric emissions, a correspondingly high rate of secondary electron ejection may also occur which may produce space charge effects and consequently further limit the linearity or increase the energy dependence of phototube response. In mixed radiation fields, the outputs produced by a given photon of radiation may mask outputs of other photons of radiation due to the different rates of secondary electron production.
Some prior phototubes have utilized high biasing or collecting fields to attempt to overcome these space charge effects. The sensitivity is then a strong function of the secondary emission coefficient which may change from detector to detector and in any one detector with time. The secondary emission coefficient may also be affected by the condition of the surface of the cathode since different surface conditions may produce different numbers of secondary electrons in a given mixed radiation field. Thus, these phototubes required precise manufacturing techniques and controls to insure a uniform surface condition. Further, since the phototube output signal includes low energy secondary electrons which require a greater time to reach the anode than high energy primary electrons, the response time of the phototube may be limited by the secondary electron transit time.